1. Field of the Invention
The present invention relates to a uranium adsorbent for use in trapping uranyl ions, and more particularly to a uranium adsorbent for use in trapping uranyl ions contained in aqueous solutions such as seawater.
2. Discussion of the Related Art
Attempts to recover uranyl ions from aqueous solutions containing uranyl ions, such as seawater and water drained from dams, have recently been made in various quarters. Uranyl ions have been attracting attention as a fuel material of the atomic energy which has accounted for an increasing proportion among the various sources of energy production these days. Thus, developed nations have been competing in research with a view to developing effective methods of recovering uranyl ions from inexhaustible seawater.
Conventional methods of trapping uranyl ions contained in an aqueous solution such as seawater include a process comprising the step of contacting an aqueous solution such as seawater with an adsorbent such as a polymer adsorbent bearing an inorganic metallic oxide (e.g. titanic acid) or a macrocyclic hexacarboxylic acid, or an amidoxime-containing adsorbent prepared by reacting a nitrile-containing polymer with hydroxylamine, and the step of eluting uranyl ions adsorbed on the adsorbent.
There are a number of known serviceable forms of amidoxime-containing adsorbents, examples of which include not only grainy and fibrous adsorbents prepared using as a starting material a nitrile-containing polymer available at a low price, but also composite adsorbents comprising an amidoxime-containing substance in powdery form incorporated into a polymer matrix.
As described above, an adsorbent containing amidoxime groups is prepared by a reaction of a polymer containing nitrile groups with hydroxylamine. It has been said that, when hydroxylamine is used in the form of an aqueous solution thereof in the above-mentioned reaction, a large number of functional groups such as hydroxamic acid groups, carboxylic amido groups and carboxylic acid groups, which take little part in adsorption of uranium, are formed as by-products through hydrolysis of amidoxime groups in addition to formation of amidoxime groups, which have heretofore been believed to take direct part in adsorption of uranium. In view of this, it has been recommended that hydroxylamine should be used in the form of a methanol solution thereof rather than an aqueous solution thereof in order to secure amidoxime groups (see Egawa, Nippon Kagaku Kaishi, 1980, p. 1767).
However, an amidoxime-containing adsorbent prepared using a methanol solution of hydroxylamine is generally poor in the capacity of adsorbing uranyl ions contained in seawater (hereinafter referred to as the "uranium adsorptivity"). A treatment of the above-mentioned amidoxime-containing adsorbent with an alkali to improve the uranium adsorptivity thereof was proposed (see Kato et al., Bulletin of the Society of Sea Water Science, Japan, Vol. 35, p. 156 (1881), and Japanese Patent Publication No. 16,812/1884). The reason for such an improvement in the uranium adsorptivity of an amidoxime-containing adsorbent through a treatment thereof with an alkali has not thoroughly been elucidated yet, but an increase in the hydrophilicity of the adsorbent through hydrolysis of nitrile groups remaining in the above-mentioned amidoxime-containing adsorbent is claimed to be responsible for the improvement (Kato et al, Nippon Kagaku Kaishi, 1982, p. 1455).